12 July 2011

App Note 2

Ha! Another classic element of the Jim Williams style: Along with more scope photos from his Tek 547, this app note includes a vacuum-tube circuit in Figure 11A. Plus, it's an Eimac 75TH (which is a gigantic bulbous triode, almost eight inches tall; not really a practical tube to use). This is Jim's practical-joker side.

This app note contains more tricks and tips for using three-terminal regulators. Figure 2 (particularly the presence of the big capacitors) supports my assertion that feedback-loop design with three-terminal regulators is hard and ad hoc because good transfer-function models aren't available. In this case, you've got to add a lot of damping to the loop to make it well behaved. Then, once you've got 100 microfarads on the output, you've got to add Q4 to maintain a quick disable. Similarly, Figure 12, showing an LT1001 precision op amp in the feedback path of an LT317AH, gives me some more feedback-induced indigestion. What's the loop gain around that loop? Yikes.

Figure 5 uses a switching regulator to control the voltage across a linear regulator. Neat trick! The best circuit is the switcher-controlled linear regulator in Figure 7, driven from AC using SCRs that is up to 85% efficient. (I must admit that I have never used an SCR in a circuit design. I am embarrassed.) Note the LM301A in an integrator topology with 100-pF compensation cap and a diode clamp on pin 8. Again, the loop compensation here is very, very conservative.

The final circuit is another 110VAC/220VAC dual voltage solution (a much better approach than Figure 5 in App Note 1). Clearly, he was grappling with this problem. Plus, another SCR!

Best quote (page AN2-5): "Because these two feedback systems are interlocked, frequency compensation can be difficult." No argument here.

UPDATE (with respect to Figure 12, and the LT1001 and LT317A):  As Joe alludes to in his comment below, there are basically two ways to build an adjustable regulator, as illustrated here (with much simplification).

The topology on the left would be totally unstable with an op amp in the feedback path, while the topology on the right might be stable with an op amp in the feedback path (depending on the unity-gain frequency of the op amp and the bandwidth of the regulator).  I was thinking that the LT317A was like the topology on the left, but looking at the datasheet, it's like the topology on the right (either way, it'd be nice to have a transfer-function model).  Thanks Joe!


Joe Sousa said...

In figure 12, the loop gain for the regulator/opamp combination is dominated by the LT1001 OPAMP A1 because the LT317AH regulator is used as a non-inverting closed loop buffer with a +1.2V offset from ADJ to OUT.
Perhaps there is an internal zero from ADJ to OUT that bypasses these pins at high frequencies. The 2k resistor driving ADJ hints at that, and it should raise the 317 impedance at higher frequencies, which will have a (hopefully calming) effect on the stability of capacitive loads.

Your wistful thoughts on designing with SCR's remind me of vertical TV sweep circuits of 30-40 years ago, which also used SCR's. Can't remember how they worked either.

I like the automatic 110/220 supply switch in Figure AN2-14 too.

Kent Lundberg said...

Good point! I'll update the post.

markrages said...

Either regulator topology will make a regulator, but only the one on the right will make a three-terminal regulator. (The one on the left requires a ground reference which would bump the pin count to 4.)